Method of electrodepositing iridium



United States Patent 3,207,680 METHOD OF ELECTRODEPOSKTENG lRlDlUM Elizabeth L. MacNamara, Philadelphia County, Pa., as-

signor to the United States of America as represented by the Secretary of the Army No Drawing. Filed May 3, 1962, tier. No. 192,289

3 Claims. (Cl. 20447) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This application is a continuation-in-part of my copending patent application, Serial No. 53,848, filed September 2, 1960, and now abandoned, and relates to the electrodeposition of iridium, and it has particular reference to an improved bath and method for its deposition on suitable base metals.

The prior art has generally disclosed broad principles for the electrodeposition of metals. However, the meager elforts to satisfactorily electrodeposit iridium have met with relatively little success. The deposits which such prior art techniques have provided generally have proved unsatisfactory due, for example, to the existence of flaws including pits, blemishes, inclusions and cracks which appear in the structure of the deposit. Furthermore, the iridium thus plated has suffered from a tendency to peel from the base metal or otherwise exhibit poor properties of adhesion and cohesion, or were deposited in discrete particles, non-metallic in appearance, and characterized .by its soft, blackish, velvety, powdery nature.

Still other undesirable characteristics of the prior art iridium electroplates are those of low density, non-uniform composition or other irregularities and imperfections. In addition, the techniques by which those electrodeposits have been accomplished are rather complex and difiicult to perform. For example, to obtain such iridium deposits it has been necessary to carefully select, combine and proportion the materials used in molten salt baths and to carefully control critical operating conditions. Despite the elaborate methods which were developed for this purpose, the aforementioned deposits resulted.

Iridium possesses excellent physical properties, including, among others, a high melting point, high density and hardness and good resistance to high temperature oxidation. Its successful electrodeposition doubtless will find widespread use in electrical contacts and the like for missile instrumentation and similar applications.

It is therefore a principal object of this invention to provide a method wherein iridium may be successfully electrodeposited directly onto various base metals.

Another object of the invention is to provide suitable bat-h composition for electrodepositing iridium which is characterized by the single addition of an acid to water.

A more specific object of the invention is to provide a method and means for obtaining an electroplate of iridium which possesses high cohesive and adhesive properties and therefore will not peel from the base metal upon which it is plated.

A further object of the invention is to provide an electrodeposit of iridium characterized by a bright, metallic, lustrous appearance, said electrodeposit being continuous and non-discrete.

A final object of the invention is to provide an electroplate which is characterized by a high melting point, high density and hardness and yet resistant to high temperature oxidation.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the specification which follows:

Through the inventive process and bath disclosed here- 3 ,207,680 Patented Sept. 21, 1965 in, it is now possible to very successfully plate iridium upon various base metals of all forms or shapes including bars, Wires, rods, plates, sheets, strips, and other products of more intricate structure.

In supplying the iridium irons necessary for the improved electroplating bath, I prefer to employ solid chloriridic acid, H IrCl containing about 41% iridium although not limited thereto. To meet most practical purposes, approximately 1 gram of iridium per liter of water may be taken as the lower limit of concentration, or approximately 2.44 grams of the aforementioned chloriridic acid. The upper concentration has been found to be approximately 100 grams of iridium or about 244 grams of the solid acid per liter of water, the preferred concentration being about 10 grams of iridium. The resulting bath at the referred concentration will have a pH of approximately 1.8, but may range between about 0.5 and 2.0, the temperature ranging between about 25 to C. depending upon the iridium content of the bath, although about 60 C. is preferred. The electrodeposition is then accomplished .by means of a current density of about 20 to 120 amps/ft. of cathode surface or of the object being plated. Preferably, the current density should be about 20 amp/ft. for the optimum iridium concentration, depending somewhat upon the specific requirements desired in the deposits.

Althought chloriridic acid is preferred, other compounds have been found to work quite admirably in electrodepositing iridium. Thus, in the generic formula where X is a univalent cation such as hydrogen, sodium or potassium, and Y is a univalent anion such as chlorine, bromine or iodine, good, dense, bright, metallic, continuous, highly adherent and mechanically resistant deposits of iridium were successfully formed. I have found that the range of concentration, temperatures, current densities and the preferred concentration of iridium ions per liter of Water, preferred temperatures and preferred current densities of these compounds are generally identical with those when chloriridic acid is used. The pH of the bath solution however will depend on the cation or anion selection for the electrolyte and needs no regulation.

The preparation of the cathode or article to be plated should be such as to provide a chemically clean surface in order to secure the desired adherence of iridium to the cathode or article. One suitable method for doing this is to make the article the anode in an electrolytic bath containing about 70% sulfuric acid and subjecting the article to this treatment for several minutes using suitable current densities. Optionally, however, other techniques of cleaning may be employed, including brushing, grinding, sandblasting, shot blasting, degreasing and the like.

Various types of articles or base members may be plated with the iridium. Copper, nickel and niobium have been found extremely amenable to adherent iridium deposits. Plain carbon steel which has been flash coated with nickel, and moybdenum, the surface of which need only be slightly etched, as, for example, by a solution consisting essentially of 1 gram potassium ferrocyanide, 10 grams potassium hydroxide and milliliters of water, may similarly be successfully plated with iridium.

To accomplish such electrodeposition of iridium, the bath solution of cbloriridic acid, for example, and water is provided in any suitable container. The cleaned objectto-be-plated is made the cathode and immersed in the plating bath. For the insoluble anode material, I prefer to use platinum, although conventional carbon anodes have been found to work satisfactorily. To operate this electroyltic bath, a suitable source of direct current is employed to produce the current densities aforedescribed.

Under optimized conditions of operation, i.e., using chloriridic acid, an electrolyte consisting essentially of about grams or iridium ions per one liter of water will result in a solution having a pH of approximately 1.8. This solution will be maintained at about 60 C., the electrodeposition being carried out at a current density of about 20 amperes per square foot of the cleaned cathode or article-to-be-plated surface which is preferably nickel while the anode is preferably platinum. The iridium will plate out with a cathode efficiency in the neighborhood of about 12.2%. The cathode efficiencies for the other compounds will vary in accordance with the compound selected and the conditions under which electrodeposition is carried out.

From the foregoing it will thus be seen that by following my inventive methods, and by employing my novel bath compositions within the broad limits hereinbefore described, I have made possible the obtainment of superior electroplated iridium deposits. The deposits are dense, continuous, non-discrete, metallic in appearance and, in general, are highly adherent and quite resistant to mechanical removal from the underlying basis metal. It is also important to note that the electroplates obtained by the invention are resistant to high temperature oxidation and the action of hot gases at high pressures, and whose surface is free of nodules, pits or blemishes.

I claim:

1. The method of electrodepositing dense, continuous, bright metallic, adherent and mechanical-removal resistant iridium directly onto a suitable metallic object selected from the group consisting of copper, nickel, niobium and molybdenum, and comprising the steps of connecting the object to be plated as a cathode in an electrolytic bath consisting of an aqueous solution of an iridium salt having the general formula where X is a univalent cation selected from the group consisting of hydrogen, sodium and potassium, Y is a univalent anion selected from the group consisting of chlorine, bromine and iodine, said iridium salt supplying to said solution iridium ions, maintaining said solution at a temperature ranging between about 25 to 90 C. and passing a direct current through said electrolytic bath to cause the electrodeposition of iridium on said object to be plated.

2. The method of electrodepositing dense, continuous, bright metallic, adherent and mechanical-removal resistant iridium directly onto a suitable metallic object selected from the group consisting of copper, nickel, niobium and molybdenum, and comprising the steps of connecting the object to be plated as a cathode in an electrolytic bath consisting of an aqueous solution of an iridium salt having the general formula where X is a univalent cation selected from the group consisting of hydrogen, sodium and potassium, Y is a univalent anion selected from the group consisting of chlorine, bromine and iodine, said iridium salt supplying to said solution iridium ions, maintaining said solution at a temperature ranging between about 25 to C. and passing an electric current through said electrolytic bath at a current density of about 20 to amperes per square foot of object surface upon which the iridium is being depositied.

3. The method of electrodepositing dense, continuous, bright metallic, adherent and mechanical-removal resistant iridium directly onto a suitable metallic object selected from the group consisting of copper, nickel, niobium and molybdenum, as a cathode in an electrolytic bath consisting of an aqueous solution of an iridium salt having the general formula where X is a univalent cation selected from the group consisting of hydrogen, sodium and potassium, Y is a univalent anion selected from the group consisting of chlorine, bromine and iodine, said iridium salt supplying to said solution iridium ions, maintaining said solution at a temperature of about 60 C. and passing an electric current through said electrolytic bath at a current density of about 20 amperes per square foot of object surface upon which the iridium is being deposited.

References Cited by the Examiner UNITED STATES PATENTS 362,257 5/87 Dudley 20447 2,416,949 3/47 Perley et a1 20447 2,451,340 10/48 Jernstedt 20447 2,497,110 2/ 5 0 Williams 20447 2,719,797 10/55 Rosenblatt 20447 OTHER REFERENCES Comprehensive Treatise on Inorganic and Theoretical Chemistry, Mellor, volume 15, Nov. 17, 1924.

WINSTON A. DOUGLAS, Primary Examiner.

JOHN R. SPECK, JOHN H. MACK, Examiners. 

1. THE METHOD OF ELECTRODEPOSITING DENSE, CONTINUOUS, BRIGHT METALLIC, ADHERENT AND MECHANICAL-REMVAL RESISTANT IRIDIUM DIRECTLY ONTO A SUITABLE METALLIC OBJECT SELECTED FROM THE GROUP CONSISTING OF COPPER, NICKEL, NIOBIUM AND MOLYBDENUM, AND COMPRISING THE STEPS OF CONNECTING THE OBJECT TO BE PLATED AS A CATHODE IN AN ELECTROLYTIC BATH CONSISTING OF AN AQUEOUS SOLUTION OF AN IRIDIUM SALT HAVING THE GENERAL FORMULA 